(V), 3°Ref

AgCl + e

® Ag°

+ Cl-

(1M KCl)


AgCl + e

® Ag°

+ Cl-

(4M KCl)


where n = 1 and the standard hydrogen electrode is the reference electrode. For process applications, the standard hydrogen electrode is not used; instead the silver-silver chloride electrode in 1 M or 4 M KCl solution is the reference electrode. Table 7.41.2 lists the half-cell potentials of these standard reference electrodes.

The following equation gives the value of the Nernst Eo term:

Eo En E rei


E°n = the standard potential of the reaction written in reduced form. Subscript n is the chemical symbol for the atom, molecule, or radical being considered.

Rewriting Equations 7.41(8) and (9) for use with the 1 M KCl electrode gives the following equations:


Note that (Emeter)Fe and (Emeter)Ce equal their respective Nernst E0 values (0.536 and 0.375 V) when the ratio of [Ox.] to [ReD.] = 1. This ratio occurs at the half-titration point.

The following equation gives the ORP value at the equivalence or endpoint of the reaction:


n1 and n2 = number of electrons in the oxidizing and reducing reactions, respectively n

For the preceding example where n = n2 = 1, E is as follows:

E =1/2 (E°Fe + E°Ce) = 1/2 (0.536 + 0.375) E = +0.456 V

The chemistry of this Fe2+/Ce4+ reaction has been simplified to demonstrate the handling of the Nernst equation and the applicable sign convention. In an actual Fe2+/Ce4+ system, the Fe2+ is usually in a highly acid solution.

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